1. Field of the Invention
The present invention relates to a tension mask assembly mounted to a Braun tube, and more particularly, to a main frame for allowing the tension mask to be under uniform tension across the whole area.
2. Description of the Related Art
In general, the tension mask is a thin plate mounted at the rear of a panel forming a screen of a Braun tube with a number of electron beam through holes at the upper surface thereof, and is subjected to tension from a frame so that electron beams passing the through holes can be effectively color selected to form an image on the screen.
The tension mask is provided in a tension mask assembly so as to be fixed within a Braun tube, in which the tension mask assembly includes a pair of sub-frames 3 formed by bending both ends of angled bars, a pair of main frames 2 welded to the upper surfaces of both ends of the sub-frames 3 with an L-shaped or triangular welded section, and the tension mask welded to the upper part of the main frames 2 with dot-shaped slots or grill-shaped electron beam through holes as shown in FIG. 1 and FIG. 2.
The tension mask assembly is assembled as follows:
As shown in FIG. 2, in the tension mask assembly, when the sub-frame 3 is inwardly bent due to a force F applied to both ends of the main frame 2, the end position of the main frame 2 welded to the upper part thereof is displaced from the initial point A1 to A3.
The tension mask 1 is welded to the upper part of the main frame 2 displaced as above. When the force F applied to the main frame is removed, the main frame 2 stresses the tension mask 1 as the sub-frame 3 returns to the original position.
However, when the force F is removed, the end position of the main frame 2 is displaced from the position A3 only to A2 thereby causing deformation as much as an interval from A1 to A2, which is up to 13 mm in a longitudinal middle portion of the main frame and about 2 mm in both longitudinal edges in a 32 inch type Braun tube so that the middle portion is strained in a greater amount compared to both edges.
Such deformation is due to the fact that both longitudinal ends of the main frame are directly supported whereas the middle portion of the main frame is strained without any restriction when the force F applied to the tension mask assembly is removed.
Therefore, in the tension mask assembly of the prior art, the middle portion of the main frame is deformed freely so that the middle portion of the tension mask welded to the main frame has a lower amount of tension compared to the edges.
As tension of the tension mask like this is varied along the longitudinal direction, the difference of the natural frequency is generated according to the longitudinal position of the tension mask, and in particular, the natural frequency is remarkably lowered in the middle portion.
Therefore, the tension mask easily resonates on sound wave or impact so that the landing position of the electron beam is varied according to the relative position change between a phosphor screen and the slot of the tension mask due to vibration of the tension mask, and a howling or color imbalance takes place on an image to degrade color purity.
Also, due to the difference of the natural frequency like this, it is difficult to adjust audible frequency range for generating vibration so as not to overlap with the natural frequency range.
Further, while the modulus of strain of a material after being tensioned by the frame should exist under the modulus of strain at the yield point in the tension mask, tension applied to the tension mask is increased due to thermal expansion of the main frame in a heat treating process so that a large amount of tension is concentrated to both ends of longer sides of the tension mask of the related art. Therefore, there is a problem that both ends of the longer sides of the tension mask may easily exceed the modulus of strain of the yield point to incur plastic deformation or break.
The present invention is proposed to solve the foregoing problems, and it is therefore an object of the invention to locally increase lower plane width of a main frame in a tension mask assembly composing a flat color Braun tube thereby to decrease strain of the main frame.
It is another object of the invention to apply uniform tension to the tension mask to prevent howling.
It is further another object of the invention to provide a tension mask assembly in which the tension mask is subjected to uniform tensile force to decrease the modulus of strain at the periphery of the tension mask thereby preventing plastic deformation or break of the tension mask in a heat treatment of the Braun tube.
Also, it is other object of the invention to restrict the main frame only to a certain portion in increase of width thereby avoiding increase of production cost.
According to an embodiment of the invention to solve the foregoing objects, it is provided a tension mask assembly comprising: a tension mask having electron beam through holes shaped as a slot or grill, a sub-frame for tensioning the tension mask, and main frames welded to the tension mask, wherein each of the main frames is bent at a middle portion in the width direction, and has a portion perpendicular to the tension mask defining a partition and another portion opposite to the tension mask defining a lower plane, wherein widths of a middle portion and both ends of the lower plane are formed in the range of the following equation:       0     less than                             w          1                -                  w          2                            w        2              ≤    1.0    ,
herein, w1 is the width of the middle portion, and W2 is the width of both ends.
According to another embodiment of the invention to solve the foregoing objects, it is provided a tension mask assembly comprising: a tension mask having electron beam through holes shaped as a slot or grill, a sub-frame for tensioning the tension mask, and main frames welded to the tension mask, wherein each of the main frames has a partition perpendicular to the tension mask, a lower plane perpendicularly bent from the partition with a certain width to be opposed to the tension mask, and a support bent from the lower plane to support the partition at the outer edge, wherein widths of a middle portion and both ends of the lower plane are formed in the range of the following equation:       0     less than                             y          1                -                  y          2                            y        2              ≤    1.0    ,
herein, y1 is the width of the middle portion, and y2 is the width of both ends.
It is preferred that widths of a middle portion and both ends of the support are formed in the range of the following equation:       0     less than                             d          1                -                  d          2                            d        2              ≤    1.0    ,
herein, d1 is the width of the middle portion, and d2 is the width of both ends.
Preferred embodiments of the invention will be described in more detail in reference to the appended drawings, wherein features and advantages of the invention will be more apparent.